Vast quantities of purchased sodium hydrosulfite solution are consumed by the kaolin industry as a reducing bleach reagent. Reductive bleaching improves the brightness and color of the clay by action on iron contaminants. Conventional reduction bleach of a kaolin slip is usually performed in in-line processing equipment including multiple static mixers through which the aqueous slip of kaolin is passed through a piping system. Typically, the slip is at about 30% solids or less to maintain fluidity. Prior to bleaching, the slip is acidified with alum, sulfuric acid or a combination thereof in order to maximize the effectiveness of the hydrosulfite bleach.
The cost of the sodium hydrosulfite bleach reagent represents a significant cost in the wet processing of kaolin ores. Not only is the chemical expensive, but it is supplied as a solution that requires refrigeration and storage under a nitrogen blanket, along with the addition of caustic stabilizers to assure activity. Dry powdered sodium hydrosulfite is stable but has been known to ignite in the presence of moisture. Thus, it is not practical for the industry to employ the powder.
Prior to the use of purchased sodium hydrosulfite solutions, zinc hydrosulfite was produced on site by most kaolin producers. The change to sodium hydrosulfite was caused by the need to remove zinc from waste water as a result of environmental regulations.
Long prior to the commercial use of preformed sodium hydrosulfite or its predecessor zinc hydrosulfite to bleach minerals such as kaolin, it was proposed (UK Patent Specification, 181,132 accepted Jun. 12, 1911) to bleach minerals, such as kaolin, by charging the mineral matter to a vessel, adding water and then introducing particulate metal and saturating the water with SO.sub.2 gas. Platinum and iron are disclosed in the UK patent but there is an expressed preference for zinc. Over a half-century ago, a modification of such procedure was proposed in which a clay slip was first treated with sulfur dioxide and then with zinc dust. See U.S. Pat. No. 2,149,506 (1939). As noted above, zinc hydrosulfite, either purchased or generated on site, eventually became an industry standard.
A return to the use of metals as reactants in hydrosulfite bleach manufacture appears in U.S. Pat. Nos. 4,076,795 (1978) and 4,157,980 (1979). These patents relate to the manufacture of reduction bleaches using iron or aluminum powders and sulfur dioxide in attempt to improve bleach systems available at that time. The process using iron (U.S. Pat. No. 4,076,795) was used commercially in a plant of the assignee and generated a sodium hydrosulfite solution as the bleach reagent. In carrying out the process, the reaction product between sulfur dioxide and iron and was converted to the sodium form, generating a hydrated iron-oxide by-product. The major difficulty in the manufacturing process was the removal of the precipitated iron oxide by-product from the bleach.
There are many incentives to use the in situ reaction of iron and sulfur dioxide to bleach clay slips instead of employing purchased sodium hydrosulfite solution. Simple calculations point to a potential to reduce chemical costs significantly, assuming that the material will have the capability of bleaching iron in an iron-stained clay. An in situ bleach also offers the potential to avoid storage problems encountered in storing purchased sodium bleach.
The long span of time subsequent to the above-cited British Specification and the present invention attests to the difficulties in using iron in an in situ bleach. The aforementioned plant scale operation using iron and sulfur dioxide in a non in situ process (which generated sodium hydrosulfite as the actual bleach reagent rather than a iron hydrosulfite) suggests that it was not obvious to use an iron salt to bleach iron-contaminated clay.
Prior to the present invention, it was not known, one way or another, whether the reaction product of metallic iron and sulfur dioxide would result in a reducing bleach capable of removing an iron stain from a kaolin. One possibility would be that the iron in the bleach reagent would actually further stain the kaolin.
Simple batch experimentation using sufficient sulfur dioxide to react with all the iron indicated that the chemistry was indeed favorable. Thus, using laboratory scale glassware, sulfur dioxide was bubbled through a fritted glass tube into a beaker containing a slip of kaolin of the type known to respond to bleaching with sodium hydrosulfite. Iron was added either before or after addition of the sulfur dioxide. The treated clay was flocced by the presence of sulfur dioxide in the slip, thus being in a condition suitable for filtration. After filtration and washing, the results were generally similar to those realized with the sodium hydrosulfite bleach.
However, efforts to translate the laboratory batch-scale in situ iron/sulfur dioxide process to a continuous commercially viable operation were beset with formidable difficulties. Generally, the problems resulted from the need to operate on a continuous basis in which large volumes of clay slips are continuously charged with relatively small amounts of iron and sulfur dioxide.
Feeding sulfur dioxide gas on a continuous basis presented a serious challenge. Equipment of the type used in treating municipal wastes or the like with chlorine gas were trailed, the gas injector being submerged in a tank holding the clay slip with added iron powder. One of the major difficulties in adapting the equipment to this new use was that the specific gravity of the iron powder (8 g/cc) was so much higher than that of the clay slip (1.22 g/cc). Consequently, the iron powder could not be suspended in the slip of clay to be bleached. It was concluded that current technology could not provide a means for controlling the extremely small flow of solid iron powder into a liquid stream of the clay feed.